In a process of manufacturing a semiconductor device, impurity (dopant) introduction is an essential process for forming a pn junction in a semiconductor wafer. The current impurity introduction is typically achieved by an ion injection technique and an annealing technique thereafter. The ion injection technique is a technology of ionizing impurities of elements such as boron (B), arsenic (As), and phosphorus (P) and physically injecting the ionized impurities into a silicon substrate through collision at high acceleration voltage (refer to Patent Document 1, for example). The injected impurities are activated through anneal treatment.
The ion injection technique, which has been conventionally and widely performed at impurity introduction, provides easiness of controlling of the injection depth and concentration of impurities. However, recent further refinement of semiconductor devices requires impurity introduction only into an extremely shallow region (at a depth of several nm or less) of the surface layer of a substrate. It is difficult with the ion injection technique to accurately inject impurities only into such an extremely shallow surface layer region. This is because the impurity injection by the ion injection technique generates crystal defects, which may increase variation in depths at which the impurities are injected into the surface layer of a semiconductor wafer, and may cause abnormal diffusion of the impurities at annealing.
In a developing technology (extremely shallow junction forming technology), a mono-molecular layer including impurities (dopants) is formed on the surface of a silicon substrate through wet treatment, and the impurities are diffused into the outermost surface layer of the substrate through heat treatment thereafter to introduce the impurities only into an extremely shallow region of the surface layer of the substrate (refer to Non Patent Document 1). Examples of such an extremely shallow junction forming technology include a technology utilizing mono layer doping (MLD). In this technology, impurities diffuse from a mono-molecular layer to a substrate, and thus are introduced only into an extremely shallow region of the surface layer of the substrate. When a mono-molecular layer including impurities is formed on a complicated irregular surface of a substrate through wet treatment, the impurities can be uniformly introduced into the entire surface.